2 BYZANTINE CHURCHES CHAP. (1) Buildings in which the ground plan is cruciform^ so that. the cross shows externally at the ground level Churches of this class are usually small3 and were probably sepulchral chapels rather than churches for public worship. A good example is the tomb of Galla Placidia at Ravenna. (2) In the second form of the Cross church the cross is enclosed within a square, and appears only above the roofs of the angle chambers. An example is seen in the late Roman tomb at Kusr en N&eijis in Eastern Palestine. In this instance the central square area is covered with a dome on continuous pendentives ; the four arms have barrel-vaultSj and the angles of the cross are occupied by small chambers, which bring the ground-plan to the square. The building is assigned to the second century3 and shows that true though continuous pendentives were known at an early datel (Fig. 8). Another example is the Praetorium at Musmiyeh3 in Syria/which probably dates from between 160 and 169 A.D. At some later time it was altered to a church, and by a curious foreshadowing of the late Byzantine plan the walls of the internal cross have entirely disappeared from the ground-plan. The dome rests on four columns placed at the inner angles of the cross3 and the vaulted cross arms rest on lintels spanning the space between the columns and the outer walls. From these three types of building are derived the various schemes on which the churches of the Byzantine Empire were planned. Of the basilican form the only example in Constantinople that retains its original plan is S. John the Baptist of the Studion (p. 56), erected c. 463 A.D. The church of SS. Sergius and Bacchus (p. 70) and the baptistery of S. Sophia (p. 78) represent respectively the two varieties of the octagonal plan. In the former the dome rests on piers surrounded by an ambulatory ; in the latter 1 Eastern Palestine Memoirs^ p. 172. A similar dome is given by Choisy, UArt de bAtir ckez les tyxantins, Plate XV. 2 De Vogtie", Syrie central^ i. p. 45, Plate VII.